Rocket engines develop a considerable heat load and the walls of the combustion chamber and the nozzle used for expanding the gas are exposed to very high temperatures. An efficient cooling is needed to avoid that the walls melt or in other ways are destroyed. In liquid propellant rocket engines; i.e., rocket engines using liquid fuel, cooling is usually achieved by leading cool fuel such as hydrogen or kerosene in channels inside the walls and thus use the fuel as cooling medium.
The heating of the construction material of the wall leads to a thermal expansion of the material. With an intense heat load on the hot side of the wall and with simultaneous cooling from inside the wall, a significant temperature gradient develops in the wall. This leads to a thermal expansion gradient that causes a considerable thermal stress within the wall, which limits the service life of rocket components such as the thrust chamber, i.e. the combustion chamber and the nozzle. The most limiting location is the inner part of the wall in the combustion chamber; i.e., the hot side of the wall located between the cooling channels and the wall surface facing the combustion chamber.
Both reusable and non-reusable rocket engines need to withstand high thermal loads. Reusable rocket engines, however, need to withstand a repeated exposure to a thermal load as they experience a plurality of launchings; that is, such engines need to have a long, low cycle fatigue life. The better the resistance to low cycle fatigue loads, the more times it can be used.
The total strain of the inner part of the wall depends on the thermal gradient through this part of the wall and also on the thermal gradient through the whole wall, from the hot side to the cool side. By lowering the strain, the service life can be extended. A low strain in the inner part of the wall also leads to a lower strain in the outer part of the wall since the forces in the walls are each others force and reaction force.
Usually the fuel is hydrogen. A complication arising when using hydrogen as a cooling medium, however, is that metallic materials are often sensitive to hydrogen exposure which commonly results in a reduced material strength. This restricts options in material choice.
Materials with high heat conductivity reduce the thermal gradient and thereby the thermal strain in the wall structure. Copper and aluminum are materials with high heat conductivity, but the use of these materials is limited since the highest allowed operation temperature may be exceeded in phases of the flight cycle where coolant is not available such as in the reentry phase. Materials with low thermal expansion also lower the thermal strain in the wall structure. It is, however, difficult to find low thermal expansion materials that also are ductile, resistant to hydrogen exposure and suitable for processing.
A number of different wall structures have been previously proposed. In one structure the cooling medium is led through tubes with a circular cross section that are welded together parallel to each other. Such a construction is flexible in a direction perpendicular to the longitudinal axis of the tubes in that the thermal expansion can be absorbed by deflexion of the tubes that can take an oval cross-sectional form. However, the construction is rigid in the axial direction of the tubes. Another drawback is that the wavy topology of the construction leads to very high temperatures at hot spots at the crests of the tubes on the hot side of the wall.
In another structure, tubes with a rectangular cross section are welded together on the cold side, the outer side, of the wall. This structure has no parts that stick out from the hot side of the wall. Further, the construction enables a distance to be formed between the tubes on the inner side of the wall during a cooling period since the tubes are joined only at the outer side of the wall. This reduces the thermal stress during cooling. However, since distances are formed between the tubes the inner wall will not be smooth which leads to an increased friction and thus a lowered average flame velocity.
Another example is a so called sandwich structure in which a primary plate is, for example by milling, provided with cooling channels and a secondary plate is welded to the primary plate as a cover onto the cooling channels. In such a construction, the inner wall is continuous in a tangential direction and therefore the structure provides very little flexibility to reduce the strain arising from the thermal expansion.
It is also known to provide the inner wall with a thermal barrier coating using a material with low heat conductivity, such as a ceramic material, to insulate the load carrying metallic structure. The low heat conductivity of this material has the effect that the temperature in the coating increases for a constant thermal load. Due to the thermal expansion the coating will be strongly loaded in compression and, together with the high thermal load, this leads to flaking of the coating. A general drawback with such thermal barrier coatings in such applications as rocket engines is that the coated component acquires an additional weight.